Thereafter, I believe MAME would have HLSL to apply interlacing filters to make the monitor scanlines less sharp and more like a true arcade monitor.
That assumption is not correct. Using 31kHz 240p on a PC CRT produces very thin, sharp scanlines. This is due to the fact PC monitors have excellent beam focus in order to produce high resolution, and the beam strikes a very fine grille or mask. The video source (your PC, here) cannot affect this sharpness. So, you'd be filtering or blurring the image on the PC side, not affecting how sharp the scanlines are.
This can work slightly with bilinear flicker filters, but those are applied *to* interlaced modes, and it's an image effect not a focus adjustment. 240p is not an interlaced mode, however, so anything you did apply would not be an interlaced filter. If you meant the filter described by hizzlekizzle in the first comment of that reddit thread, sure, that would work, and cut your screen to 50% brightness instantly. But then you're not running 240p anymore either, and your post seemed to be saying you want to. Either way this would not affect scanline sharpness, i wouldn't think.
EDIT: TBH i'd give all the optiosn a go and see what you like the look of. I need to get around to trying 31kHz 240p one day, i'm sure it must be very nice in some ways.